- Email: [email protected]
Nonlinear optics
capacitance. With regard to the 42 ns risetime, the variable delay was adjusted to about 100 ns. A qualitative pulse schedule of optical and electrical signals is shown in Fig. 3, clearly demonstrating the important advantage of the optical-switching technique. By considering the laser-excited gap and shunt regions as lumped conductances,3z4 a quantitative analysis could be found to precalculate the output pulse shape of the switched signal.’ In conclusion, when using the device and technique reported in this paper, laser-controlled switching down to the picosecond range no longer depends on carrier recombination or diffusion. The optoelectronic switch is inoperative, however, until the optically generated electrons and holes have either recombined or been swept away by carrier diffusion. Detailed investigations on recombination mechanisms have shown that excitation levels such as those produced by picosecond megawatt pulses will cause a decay of photoconductivity which is mainly controlled Auger recombination,2 while excitation
by band-to-band
levels such as those produced by a 40 W GaAs laser diode will effect a diffusioncontrolled decay of photoconductivity.g Consequently, the repetition rate of laser-controlled switches is limited to excitation-dependent values of 0.1-I MHz for Si substrates if undoped high-resistivity material is used,7 and up to 1 GHz for Cr doped semi-insulating GaAs substrates.” State of the art A review of the new technique would remain incomplete without reporting the present state of the art which may be summed up as follows: laser-controlled switching with 95% power transmission and switching times (rise-time, fall-time) of about 1O-l 5 ps in 1O4n cm silicon could be obtained by using 1 MW pulses of 5 ps duration produced by a modelocked Nd:glass laser.17’ Furthermore, 50% power transmission and switching times of about 1 ns in 6 x lo3 s2 cm
silicon have been obtained by using 10 kW pulses of 5 ns duration produced by a Rhodamine 6 G dye laser pumped with a nitrogen laser. ” Finally, 50% power transmission and switching times of about 70-100 ns in 4 x 103fi cm silicon have been obtained by using only 40 W pulses of 70 ns duration generated from a GaAs laser diode.’ In all these cases, the maximum repetition rate ranged from 0.1 to 1 MHz. Higher repetition rates, of up to 1 GHz, have recently been obtained by using a slab of 1O652 cm GaAs as the switching element in a charged line pulser.” Since the carrier lifetime in such a GaAs slab was less than 100 ps, the device only required an optical pulse to turn on. It turned off automatically due to fast relaxation. The maximum excess carrier density was 2.5 x 10’9cm-3, generated by a 10 c1.J pulse at X = 0.53 pm. Summary In comparison with conventional electronic switching devices, the optical technique reported in this article offers a significant improvement in both speed and power handling. By using special microstrip structures, a fast broad-band switch working up to the GHz range has been obtained. Apart from these capabilities, the application of picosecond optical pulses ensures high-speed switching without damage to the material. Furthermore, since the controlling signals are optical pulses, isolation between the electrical signal and the switching pulses is inherently high. In conclusion, laser-controlled switching may really be considered as a new field of laser application and, from this aspect, should stimulate further work on optics and laser technology, especially on fast high-power laser diodes to reduce the hardware of the controlling optical arrangement. Acknowledgement The author wishes to thank Professor Dr H. Brand for continuing interest and encouragement, particularly during the time that the initial experiments on the new field were performed.
References LeFur, P., Auston, D.H. A kilovolt picosecond optoelectronic switch and Pockels cell ApplPbys Lett 28 (1976) 21-23 Johnson, A.M., Auston, D.H. Microwave switching by picosecond photoconductivity IEEE Tram QE-11 (1975) 283-287 Platte, W. Lichtgesteuerte Mikrowellenmodulatoren in Microstrip-Technik Nachrichtenrechn Zeitschrif? 29 (1976) 737-740
4 5
Platte, W., Appelhans, G. Optoelectronic gating of microwave signals using a silicon microstrip shunt modulator Elecfron Lett 12 (1976) 210-271 Platte, W. An optoelectronic microwave modulator using a resonance matched microstrip radiation sensor Archiv fur Elektronik und Ubertragungstechnik
cc t___
b
_ tr
‘f
lL +
Cpmpressed time scale
t-
Fig. 3 Pulse schedule of optical and electrical signals. a - Switching optical pulses (y = 0.3); b - switched and detected microwave signal. The broken curve gives the shape of the output signal if the turn-off pulse is missing, which will cause relatively long decay times. R, is the residual power transmission due to the gap capacitance. Risetime t,, delay time td and falltime tf are in the ns-ps range depending on the applied laser systems
42
30 (1976) 481-484
Auston, D.H. Picosecond optoelectronic switching and gating in siliconApp[Phys Lett 26 (1975) 101-103 Platte, W. High-speed optoelectronic switching in silicon gapshunt microstrip structures Elecrron Left 12 (1976) 437-438 Platte, W. Pulse shaping by laser-excited solid-state plasmas in silicon Electron Lett 12 (1976) 631-633 Platte, W. Diffusion-controlled decay of laser-excited photoconductivity in optoelectronic switches Electron Lett 13 (1977) 321-323 10 Lee, Ch.H. Picosecond optoelectronic switching in GaAs Appl 11
Phys Lett 30 (1977) 84-86 Castagne, R., Laval, S., Lava& R Picosecond l-wavelength optoelectronic gate Electron Lett 12 (1976) 438-439
OPTICS AND LASER TECHNOLOGY.
FEBRUARY
1978
CONFERENCES Association for high speed photography autumn conference Central
Electricity
Research Laboratory,
Leatherhead,
More than 80 members attended the conference at CERL, Leatherhead with the kind permission of the Director, Dr Chester and the cooperation of Mr R. Knight. We were welcomed in their excellent conference room by Dr M. Moore, Head of Thermodynamics who spoke of the other wide research interests of the Laboratory, basically in the development of electrical generation but from virtually any possible mechnism. High speed photography has been an invaluable tool, particularly in early stages of research, to gain a simpler and clearer understanding of complex processes. G. Foster of J. Hadland (PI), who chaired the morning session, had been the prime organizer and paper chairman for this occasion. R. Knight began by describing the ‘flow visualization techniques used at CEGB Laboratories’, reviewing the available techniques especially for the measurement of flow rates and temperatures. He showed a strong belief in simple, effective systems and outlined their use of shadowgraph, schlieren and interferometric systems for the study of power station buildings, smoke trials, ‘rain’ from cooling towers and liquid flow in heat exchangers. His numerous examples included a good colour schlieren and some fine high voltage discharge studies from a rotating mirror camera in the lo6 pps range. Dr J. Field of the Cavendish Laboratory, Cambridge spoke of an international study of water-jet cutting in mining in which he did the high speed photography and an American colleague (Dr Lessor) did the mathematical analysis. His main instrument was the’lmacon camera, which was used at various speeds from S x lo4 up to lo6 pictures per second. Other liquids were tested but water was most effective and economic. The pictures confirmed the theory evolved, and showed that subsonic jet velocities were effective and that efficiency did not improve until the velocity was considerably supersonic. Pulsed jets showed advantages and most material was removed when the jet impacted on water-filled holes. The last paper before lunch was again from CERL on ‘Axial view in two phase flow’ by S.A. Fisher. Examples of two-phase flow could be two different liquids or water and steam in the heat exchangers of power stations. In coal fired stations, the pipes in the furnace are mainly very long and vertical while in nuclear systems they are horizontal and both have U-bends at junctions. This paper outlined an optical system which was built into the pipes so that pictures could be taken along the flow axis. By comparative studies without this system, the amount of flow interference by the measurement
OPTICS AND LASER TECHNOLOGY.
FEBRUARY
1978
Surrey,
UK,1 1 October
1977
is, in many cases, minimal and the studies are therefore relevant. Droplet formation in the flow was recorded showing unsuspected mechanisms and valuable information about cross flows at U-bends and the degree of possible drying out of some surfaces was obtained. Techniques used cameras operating at SO0 and 2500 pps. After lunch G.H. Lunn (AWRE Aldermaston) took the chair and introduced by L. Hallett (the National delegate) who gave details of the next international conference in Tokyo (August 1978). Then E.J. Bals (Micron Sprayers) spoke about ‘High speed photographs of liquid sprays and atomization.’ He pointed out that in pest control the efficiency of liquid spraying could be as bad as 1 : log, that is one effective droplet on an insect and 10’ wasted and adding to contamination. Their aim is to produce 20 pm diameter droplets, one of which is a killing dose for insects. Any droplets of 40 pm requires eight times more insecticide so that the gains of good atomization are obvious, From high speed flash and tine studies of their toothed-wheel sprayer, they have been able to select the optimum tooth size, shape and rotor speed. They now have a better understanding of droplet production and are achieving large economies in liquid used, saving money and reducing contamination. ‘High speed photography in jet turbulence research’ was the subject of D.R.J. Baxter of Southampton University. The original project began from a meter/computer technique and photographs were requested only as illustrations. The information so readily obtained by high-speed photo techniques caused them to become a principal research tool. The camera speeds are relatively modest, 200 pps, and the hot wire anemometer system in use became the camera trigger. They used side and axial views and introduced pulsed hydrogen bubbles to identify a chosen plane in the flow. This was a most effective technique and added an attractive pattern structure to the records. The interruptions to the added bubbles created dark lines at regular known intervals. A change of subject came from S. Young of the Royal College of Surgeons who has been making ‘High speed intermittent tine-angiography of the cat ocular fundus’. Dye is introduced into the carotid artery of a cat and then tine studies are made via a modified opthalmoscope (fundus) camera viewing the inner back surface of the cat’s eye and recording the passage of the dye in the arteries and veins. The reason is that many ocular diseases are due to bad blood flow and an understanding of blood
43
by band-to-band
levels such as those produced by a 40 W GaAs laser diode will effect a diffusioncontrolled decay of photoconductivity.g Consequently, the repetition rate of laser-controlled switches is limited to excitation-dependent values of 0.1-I MHz for Si substrates if undoped high-resistivity material is used,7 and up to 1 GHz for Cr doped semi-insulating GaAs substrates.” State of the art A review of the new technique would remain incomplete without reporting the present state of the art which may be summed up as follows: laser-controlled switching with 95% power transmission and switching times (rise-time, fall-time) of about 1O-l 5 ps in 1O4n cm silicon could be obtained by using 1 MW pulses of 5 ps duration produced by a modelocked Nd:glass laser.17’ Furthermore, 50% power transmission and switching times of about 1 ns in 6 x lo3 s2 cm
silicon have been obtained by using 10 kW pulses of 5 ns duration produced by a Rhodamine 6 G dye laser pumped with a nitrogen laser. ” Finally, 50% power transmission and switching times of about 70-100 ns in 4 x 103fi cm silicon have been obtained by using only 40 W pulses of 70 ns duration generated from a GaAs laser diode.’ In all these cases, the maximum repetition rate ranged from 0.1 to 1 MHz. Higher repetition rates, of up to 1 GHz, have recently been obtained by using a slab of 1O652 cm GaAs as the switching element in a charged line pulser.” Since the carrier lifetime in such a GaAs slab was less than 100 ps, the device only required an optical pulse to turn on. It turned off automatically due to fast relaxation. The maximum excess carrier density was 2.5 x 10’9cm-3, generated by a 10 c1.J pulse at X = 0.53 pm. Summary In comparison with conventional electronic switching devices, the optical technique reported in this article offers a significant improvement in both speed and power handling. By using special microstrip structures, a fast broad-band switch working up to the GHz range has been obtained. Apart from these capabilities, the application of picosecond optical pulses ensures high-speed switching without damage to the material. Furthermore, since the controlling signals are optical pulses, isolation between the electrical signal and the switching pulses is inherently high. In conclusion, laser-controlled switching may really be considered as a new field of laser application and, from this aspect, should stimulate further work on optics and laser technology, especially on fast high-power laser diodes to reduce the hardware of the controlling optical arrangement. Acknowledgement The author wishes to thank Professor Dr H. Brand for continuing interest and encouragement, particularly during the time that the initial experiments on the new field were performed.
References LeFur, P., Auston, D.H. A kilovolt picosecond optoelectronic switch and Pockels cell ApplPbys Lett 28 (1976) 21-23 Johnson, A.M., Auston, D.H. Microwave switching by picosecond photoconductivity IEEE Tram QE-11 (1975) 283-287 Platte, W. Lichtgesteuerte Mikrowellenmodulatoren in Microstrip-Technik Nachrichtenrechn Zeitschrif? 29 (1976) 737-740
4 5
Platte, W., Appelhans, G. Optoelectronic gating of microwave signals using a silicon microstrip shunt modulator Elecfron Lett 12 (1976) 210-271 Platte, W. An optoelectronic microwave modulator using a resonance matched microstrip radiation sensor Archiv fur Elektronik und Ubertragungstechnik
cc t___
b
_ tr
‘f
lL +
Cpmpressed time scale
t-
Fig. 3 Pulse schedule of optical and electrical signals. a - Switching optical pulses (y = 0.3); b - switched and detected microwave signal. The broken curve gives the shape of the output signal if the turn-off pulse is missing, which will cause relatively long decay times. R, is the residual power transmission due to the gap capacitance. Risetime t,, delay time td and falltime tf are in the ns-ps range depending on the applied laser systems
42
30 (1976) 481-484
Auston, D.H. Picosecond optoelectronic switching and gating in siliconApp[Phys Lett 26 (1975) 101-103 Platte, W. High-speed optoelectronic switching in silicon gapshunt microstrip structures Elecrron Left 12 (1976) 437-438 Platte, W. Pulse shaping by laser-excited solid-state plasmas in silicon Electron Lett 12 (1976) 631-633 Platte, W. Diffusion-controlled decay of laser-excited photoconductivity in optoelectronic switches Electron Lett 13 (1977) 321-323 10 Lee, Ch.H. Picosecond optoelectronic switching in GaAs Appl 11
Phys Lett 30 (1977) 84-86 Castagne, R., Laval, S., Lava& R Picosecond l-wavelength optoelectronic gate Electron Lett 12 (1976) 438-439
OPTICS AND LASER TECHNOLOGY.
FEBRUARY
1978
CONFERENCES Association for high speed photography autumn conference Central
Electricity
Research Laboratory,
Leatherhead,
More than 80 members attended the conference at CERL, Leatherhead with the kind permission of the Director, Dr Chester and the cooperation of Mr R. Knight. We were welcomed in their excellent conference room by Dr M. Moore, Head of Thermodynamics who spoke of the other wide research interests of the Laboratory, basically in the development of electrical generation but from virtually any possible mechnism. High speed photography has been an invaluable tool, particularly in early stages of research, to gain a simpler and clearer understanding of complex processes. G. Foster of J. Hadland (PI), who chaired the morning session, had been the prime organizer and paper chairman for this occasion. R. Knight began by describing the ‘flow visualization techniques used at CEGB Laboratories’, reviewing the available techniques especially for the measurement of flow rates and temperatures. He showed a strong belief in simple, effective systems and outlined their use of shadowgraph, schlieren and interferometric systems for the study of power station buildings, smoke trials, ‘rain’ from cooling towers and liquid flow in heat exchangers. His numerous examples included a good colour schlieren and some fine high voltage discharge studies from a rotating mirror camera in the lo6 pps range. Dr J. Field of the Cavendish Laboratory, Cambridge spoke of an international study of water-jet cutting in mining in which he did the high speed photography and an American colleague (Dr Lessor) did the mathematical analysis. His main instrument was the’lmacon camera, which was used at various speeds from S x lo4 up to lo6 pictures per second. Other liquids were tested but water was most effective and economic. The pictures confirmed the theory evolved, and showed that subsonic jet velocities were effective and that efficiency did not improve until the velocity was considerably supersonic. Pulsed jets showed advantages and most material was removed when the jet impacted on water-filled holes. The last paper before lunch was again from CERL on ‘Axial view in two phase flow’ by S.A. Fisher. Examples of two-phase flow could be two different liquids or water and steam in the heat exchangers of power stations. In coal fired stations, the pipes in the furnace are mainly very long and vertical while in nuclear systems they are horizontal and both have U-bends at junctions. This paper outlined an optical system which was built into the pipes so that pictures could be taken along the flow axis. By comparative studies without this system, the amount of flow interference by the measurement
OPTICS AND LASER TECHNOLOGY.
FEBRUARY
1978
Surrey,
UK,1 1 October
1977
is, in many cases, minimal and the studies are therefore relevant. Droplet formation in the flow was recorded showing unsuspected mechanisms and valuable information about cross flows at U-bends and the degree of possible drying out of some surfaces was obtained. Techniques used cameras operating at SO0 and 2500 pps. After lunch G.H. Lunn (AWRE Aldermaston) took the chair and introduced by L. Hallett (the National delegate) who gave details of the next international conference in Tokyo (August 1978). Then E.J. Bals (Micron Sprayers) spoke about ‘High speed photographs of liquid sprays and atomization.’ He pointed out that in pest control the efficiency of liquid spraying could be as bad as 1 : log, that is one effective droplet on an insect and 10’ wasted and adding to contamination. Their aim is to produce 20 pm diameter droplets, one of which is a killing dose for insects. Any droplets of 40 pm requires eight times more insecticide so that the gains of good atomization are obvious, From high speed flash and tine studies of their toothed-wheel sprayer, they have been able to select the optimum tooth size, shape and rotor speed. They now have a better understanding of droplet production and are achieving large economies in liquid used, saving money and reducing contamination. ‘High speed photography in jet turbulence research’ was the subject of D.R.J. Baxter of Southampton University. The original project began from a meter/computer technique and photographs were requested only as illustrations. The information so readily obtained by high-speed photo techniques caused them to become a principal research tool. The camera speeds are relatively modest, 200 pps, and the hot wire anemometer system in use became the camera trigger. They used side and axial views and introduced pulsed hydrogen bubbles to identify a chosen plane in the flow. This was a most effective technique and added an attractive pattern structure to the records. The interruptions to the added bubbles created dark lines at regular known intervals. A change of subject came from S. Young of the Royal College of Surgeons who has been making ‘High speed intermittent tine-angiography of the cat ocular fundus’. Dye is introduced into the carotid artery of a cat and then tine studies are made via a modified opthalmoscope (fundus) camera viewing the inner back surface of the cat’s eye and recording the passage of the dye in the arteries and veins. The reason is that many ocular diseases are due to bad blood flow and an understanding of blood
43